1. Field of the Invention
The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery using the same, and more specifically, to an electrolyte for a lithium-sulfur battery prepared by mixing at least two solvents with different sulfur solubilities, and another solvent with a high dielectric constant and a high viscosity, and a lithium-sulfur battery using the same.
2. Description of the Related Art
Due to the rapid development of portable electronic equipment, there are growing demands for secondary batteries. Recently, coinciding with the trend toward compact, thin, small and light portable electronic equipment, there is growing need for a battery with a high energy density. Thus, it is necessary to develop a battery with good safety, economy, and which is also environmentally-friendly.
Of the batteries satisfying the above requirements, a lithium-sulfur battery is the most useful in terms of energy density. The energy density of lithium is 3830 mAh/g used as a negative active material, and the energy density of sulfur (S8) used as a positive active material is 1675 mAh/g. These active materials are both cheap and environmentally friendly. However, there is still little widespread use of the lithium-sulfur battery system.
The reason as to why there is still little widespread use of the lithium-sulfur battery is because of the amount of sulfur used in electrochemical reduction/oxidation (redoxidation) in a battery as compared to the amount of introduced sulfur is very low. That is to say, the sulfur utilization is very low so that there is very low battery capacity when the sulfur is used as active material.
In addition, during redoxidation, the sulfur leaks into the electrolyte so that the cycle life of a battery deteriorates. Also, when a proper electrolyte is not selected, the lithium sulfide (Li2S) (i.e., the reduced material of sulfur) is deposited so that it can no longer participate in the electrochemical reaction.
U.S. Pat. No. 5,961,672 discloses a mixed electrolyte solution of 1,3-dioxolane/diglyme/sulforane/dimethoxyethane in a ratio of 50/20/10/20 comprising 1M of LiSO3CF3 in order to improve the cycle life and safety, and a negative electrode of lithium metal coated with a polymer.
U.S. Pat. Nos. 5,523,179, 5,814,420, and 6,030,720 suggest technical improvements to solve the above-noted problems. U.S. Pat. No. 6,030,720 discloses a mixed solvent comprising a main solvent having a general formula R1(CH2CH2O)nR2 (where n ranges between 2 and 10, and R1 and R2 are different or identical alkyl or alkoxy groups) and a cosolvent having a donor number of at least about 15. In addition, the above patent uses a liquid electrolyte solvent comprising at least one donor solvent such as a crown ether or a cryptand, and eventually the electrolyte turns to a catholyte after discharging. The patent discloses that the separation distance, defined as the boundaries of the region where the cell catholyte resides, is less than 400 μm.
Meanwhile, there is still a need to solve the problem of the reduced cycle life of the battery caused by using lithium metal as a negative electrode. The reason for the cycle life deterioration is that as the charging/discharging cycles are repeated, dendrite that is formed from the deposition of metal lithium at the surface of the lithium negative electrode grows and reaches to the surface of the positive electrode so that it causes a short circuit. In addition, the lithium corrosion due to a reaction of the lithium surface and the electrolyte occurs to reduce the battery capacity.
In order to solve these problems, U.S. Pat. Nos. 6,017,651, 6,025,094, and 5,961,672 disclose a technique of forming a protective layer on the surface of the lithium electrode. The requirements for the protective layer to work well are the free transfer of the lithium ions and the inhibition of contact between the lithium and the electrolyte. However, the conventional methods of forming the protective layer have some problems. Most of the lithium protective layers are formed by the reaction of an additive in the electrolyte and the lithium after fabricating a battery. However, since this method does not form a dense layer, a lot of the electrolyte permeates the protective layer and contacts the lithium metal.
Also, another conventional method includes forming a lithium nitride (Li3N) layer on the surface of the lithium by reacting nitrogen plasma at the surface of lithium. However, this method also has problems in that the electrolyte permeates through the grain boundary, the lithium nitride layer is likely to decompose due to moisture, and the potential window is very low (0.45V) so that it is difficult to use practically.